### SAS Data Analysis Examples Zero-Inflated Poisson Regression

Zero-inflated poisson regression is used to model count data that has an excess of zero counts. Further, theory suggests that the excess zeros are generated by a separate process from the count values and that the excess zeros can be modeled independently.  Thus, the zip model has two parts, a poisson count model and the logit model for predicting excess zeros. You may want to review these Data Analysis Example pages, Poisson Regression and Logit Regression.

Please Note: The purpose of this page is to show how to use various data analysis commands. It does not cover all aspects of the research process which researchers are expected to do. In particular, it does not cover data cleaning and verification, verification of assumptions, model diagnostics and potential follow-up analyses.

#### Examples of zero-inflated Poisson regression

Example 1.  School administrators study the attendance behavior of high school juniors over one semester at two schools.  Attendance is measured by number of days of absent and is predicted by gender of the student and standardized test scores in math and language arts.  Many students have no absences during the semester.

Example 2. The state wildlife biologists want to model how many fish are being caught by fishermen at a state park. Visitors are asked whether or not they have a camper, how many people were in the group, were there children in the group and how many fish were caught. Some visitors do not fish, but there is no data on whether a person fished or not. Some visitors who did fish did not catch any fish so there are excess zeros in the data because of the people that did not fish.

#### Description of the data

Let's pursue Example 2 from above using the dataset fish.sas7bdat

We have data on 250 groups that went to a park.  Each group was questioned about how many fish they caught (count), how many children were in the group (child), how many people were in the group (persons), and whether or not they brought a camper to the park (camper).

In addition to predicting the number of fish caught, there is interest in predicting the existence of excess zeros, i.e., the zeroes that were not simply a result of bad luck fishing. We will use the variables child, persons, and camper in our model. Let's look at the data.

proc means data = fish mean std min max var;
var count child persons;
run;

The MEANS Procedure

Variable            Mean         Std Dev         Minimum         Maximum        Variance
----------------------------------------------------------------------------------------
count          3.2960000      11.6350281               0     149.0000000     135.3738795
child          0.6840000       0.8503153               0       3.0000000       0.7230361
persons        2.5280000       1.1127303       1.0000000       4.0000000       1.2381687
----------------------------------------------------------------------------------------

proc univariate data = fish noprint;
histogram count / midpoints = 0 to 50 by 1 vscale = count ;
run;

proc freq data = fish;
tables camper;
run;

The FREQ Procedure

Cumulative    Cumulative
camper    Frequency     Percent     Frequency      Percent
-----------------------------------------------------------
0         103       41.20           103        41.20
1         147       58.80           250       100.00

#### Analysis methods you might consider

Below is a list of some analysis methods you may have encountered. Some of the methods listed are quite reasonable while others have either fallen out of favor or have limitations.

• Zero-inflated Poisson Regression - The focus of this web page.
• Zero-inflated Negative Binomial Regression - Negative binomial regression does better with over dispersed data, i.e. variance much larger than the mean.
• Ordinary Count Models - Poisson or negative binomial models might be more appropriate if there are no excess zeros.
• OLS Regression - You could try to analyze these data using OLS regression. However, count data are highly non-normal and are not well estimated by OLS regression.

#### SAS zero-inflated Poisson regression analysis using proc genmod

If you are using SAS version 9.2 or higher, you can run a zero-inflated Poisson model using proc genmod.

proc genmod data = fish;
class camper;
model count = child camper /dist=zip;
zeromodel persons /link = logit ;
run;

The GENMOD Procedure

Model Information

Data Set                         WORK.FISH    Written by SAS

Distribution          Zero Inflated Poisson
Dependent Variable                    count

Number of Observations Used         250

Class Level Information

Class       Levels    Values

camper           2    0 1

Criteria For Assessing Goodness Of Fit

Criterion                     DF           Value        Value/DF

Deviance                               2063.2168
Scaled Deviance                        2063.2168
Pearson Chi-Square           245       1543.4597          6.2998
Scaled Pearson X2            245       1543.4597          6.2998
Log Likelihood                          774.8999
Full Log Likelihood                   -1031.6084
AIC (smaller is better)                2073.2168
AICC (smaller is better)               2073.4627
BIC (smaller is better)                2090.8241

Algorithm converged.

Analysis Of Maximum Likelihood Parameter Estimates

Standard     Wald 95% Confidence          Wald
Parameter         DF    Estimate       Error           Limits           Chi-Square    Pr > ChiSq

Intercept          1      2.4319      0.0413      2.3510      2.5128       3472.23        <.0001
child              1     -1.0428      0.1000     -1.2388     -0.8469        108.78        <.0001
camper       0     1     -0.8340      0.0936     -1.0175     -0.6505         79.35        <.0001
camper       1     0      0.0000      0.0000      0.0000      0.0000           .           .
Scale              0      1.0000      0.0000      1.0000      1.0000

NOTE: The scale parameter was held fixed.

Analysis Of Maximum Likelihood Zero Inflation Parameter Estimates

Standard     Wald 95% Confidence          Wald
Parameter    DF    Estimate       Error           Limits           Chi-Square    Pr > ChiSq

Intercept     1      1.2974      0.3739      0.5647      2.0302         12.04        0.0005
persons       1     -0.5643      0.1630     -0.8838     -0.2449         11.99        0.0005



The output begins with a summary of the model and the data.  This is followed by a list of goodness of fit statistics.

The next block of output includes parameter estimates from the count portion of the model.  It also includes the standard errors, Wald 95% confidence intervals, Wald Chi-square statistics, and p-values for the parameter estimates.

The last block of output corresponds to the zero-inflation portion of the model.  This is a logistic model predicting the zeroes.  The output includes parameter estimates for the inflation model predictors and their standard errors, Wald 95% confidence intervals, Wald Chi-square statistics, and p-values.

All of the predictors in both the count and inflation portions of the model are statistically significant. This model fits the data significantly better than the null model, i.e., the intercept-only model. To show that this is the case, we can run the null model (a model without any predictors) and compare the null model with the current model using chi-squared test on the difference of log likelihoods.

proc genmod data = fish;
model count =  /dist=zip;
zeromodel  / link = logit ;
run;
The GENMOD Procedure

Model Information

Data Set                         WORK.FISH    Written by SAS

Distribution          Zero Inflated Poisson
Dependent Variable                    count

Number of Observations Used         250

Criteria For Assessing Goodness Of Fit

Criterion                     DF           Value        Value/DF

Deviance                               2254.0459
Scaled Deviance                        2254.0459
Pearson Chi-Square           248       1918.7890          7.7371
Scaled Pearson X2            248       1918.7890          7.7371
Log Likelihood                          679.4854
Full Log Likelihood                   -1127.0229
AIC (smaller is better)                2258.0459
AICC (smaller is better)               2258.0945
BIC (smaller is better)                2265.0888

Algorithm converged.

Analysis Of Maximum Likelihood Parameter Estimates

Standard     Wald 95% Confidence          Wald
Parameter    DF    Estimate       Error           Limits           Chi-Square    Pr > ChiSq

Intercept     1      2.0316      0.0349      1.9631      2.1000       3388.16        <.0001
Scale         0      1.0000      0.0000      1.0000      1.0000

NOTE: The scale parameter was held fixed.

Analysis Of Maximum Likelihood Zero Inflation Parameter Estimates

Standard     Wald 95% Confidence          Wald
Parameter    DF    Estimate       Error           Limits           Chi-Square    Pr > ChiSq

Intercept     1      0.2728      0.1277      0.0225      0.5232          4.56        0.0327


The log likelihoods for the full model and null mode are -1031.6084 and -1127.0229, respectively. The chi-squared value is 2*( -1031.6084 - -1127.0229) = 190.829. Since we have three predictor variables in the full model, the degrees of freedom for the chi-squared test is 3. This yields a p-value <.0001.  Thus, our overall model is statistically significant.

We may want to compare the current zero-inflated poisson model with the plain poisson model, which can be done with the Vuong test. Currently, the Vuong test is not a standard part of proc genmod, but a macro program that performs the Vuong test is available from SAS here.  Usage of the macro program requires the %include statement, in which we list the location of the macro. This macro program takes quite a few arguments, as shown below. We rerun the models to get produce these required input arguments. With the zero-inflated poisson model, there are total of five regression parameters which includes the intercept, the regression coefficients for child and camper for the poisson portion of the model as well as the intercept and regression coefficient for persons. The plain poisson regression model has a total of three regression parameters. The scale parameter is the dispersion parameter from each corresponding model, and for our poisson model is fixed at 1.


%inc "C:\Users\ALIN\Documents\My SAS Files\vuong.sas";
proc genmod data = fish order=data;
class camper;
model count = child camper /dist=zip;
zeromodel persons;
output out=outzip pred=predzip pzero=p0;
store m1;
run;
proc genmod data = outzip order=data;
class camper;
model count = child camper /dist=poi;
output out=out pred=predpoi;
run;
%vuong(data=out, response=count,
model1=zip, p1=predzip, dist1=zip, scale1=1.00, pzero1=p0,
model2=poi, p2=predpoi, dist2=poi, scale2=1.00,
nparm1=3,   nparm2=2)

The Vuong Macro
Model Information

Data Set                         out
Response                         count
Number of Observations Used      250
Model 1                          zip
Distribution                  ZIP
Predicted Variable            predzip
Number of Parameters          3
Scale                         1.00
Zero-inflation Probability    p0
Log Likelihood                -1031.6084
Model 2                          poi
Distribution                  POI
Predicted Variable            predpoi
Number of Parameters          2
Scale                         1.00
Log Likelihood                -1358.5929

Vuong Test
H0: models are equally close to the true model
Ha: one of the models is closer to the true model

Preferred
Vuong Statistic            Z    Pr>|Z|      Model

Clarke Sign Test
H0: models are equally close to the true model
Ha: one of the models is closer to the true model

Preferred
Clarke Statistic           M    Pr>=|M|      Model



For the Vuong test, a significant z indicates that the zero-inflated model is better. Here we see that the preferred model is a zero-inflated poisson model over a regular poisson model. The positive values of the z statistics for Vuong test indicate that it is the first model, the zero-inflated poisson model, which is closer to the true model.

We can use the estimate statement to help understand our model. We will compute the expected counts for the categorical variable camper while holding the continuous variable child at its mean value using the atmeans option, as well as calculate the predicted probability that an observation came from the zero-generating process.  In the estimate statement, we provide values at which to evaluate each coefficient for both the poisson model and the zero-inflation model.  The sets of coefficients of the two models are separated by the @ZERO keyword.

proc genmod data = fish;
class camper;
model count = child camper /dist=zip;
zeromodel persons /link = logit ;
estimate "camper = 0" intercept 1 child .684 camper 1 0 @ZERO intercept 1 persons 2.528;
estimate "camper = 1" intercept 1 child .684 camper 0 1 @ZERO intercept 1 persons 2.528;
run;

Contrast Estimate Results

Mean            Mean              L'Beta    Standard
Label                          Estimate      Confidence Limits     Estimate       Error     Alpha

camper = 0                       2.4220      1.9724      2.9741      0.8846      0.1048      0.05
camper = 0 (Zero Inflation)      0.4677      0.3838      0.5536     -0.1292      0.1756      0.05
camper = 1                       5.5768      4.8823      6.3701      1.7186      0.0679      0.05
camper = 1 (Zero Inflation)      0.4677      0.3838      0.5536     -0.1292      0.1756      0.05

Contrast Estimate Results

L'Beta             Chi-
Label                            Confidence Limits     Square    Pr > ChiSq

camper = 0                       0.6792      1.0899     71.28        <.0001
camper = 0 (Zero Inflation)     -0.4735      0.2150      0.54        0.4619
camper = 1                       1.5856      1.8516    641.42        <.0001
camper = 1 (Zero Inflation)     -0.4735      0.2150      0.54        0.4619



In the Mean Estimate column, we find predicted counts of fish from the poisson model, ignoring the zero-inflation model, for both camper = 0 and camper = 1, as well as the predicted probability of belonging to the zero-generating process from the zero-inflation model.  The zero-inflation model does not include camper as a predictor, so the probability of zero for both zero-inflation models is the same.  To get the expected counts of fish from the mixture of the two models, simply multiply the expected counts from the poisson model by the probability of getting a non-zero from the zero-inflation model (1 - p(zero)).  Thus, the expected counts of fish for camper = 0 and camper = 1 including zero-inflation are 2.422*(1-.04677) = 1.289 and 5.5768*(1-.04677) = 2.968, respectively.

#### SAS zero-inflated Poisson analysis using proc countreg

Proc countreg is another option for running a zero-inflated Poisson regression in SAS (again, version 9.2 or higher).  This procedure allows for a few more options specific to count outcomes than proc genmod.  The proc countreg code for the original model run on this page appears below.  We indicate method = qn to specify the quasi-Newton optimization process that matches the proc genmod results.

proc countreg data = fish method = qn;
class camper;
model count = child camper / dist= zip;
zeromodel count ~ persons;
run;

The COUNTREG Procedure

Class Level Information

Class       Levels    Values

camper           2    0 1

Model Fit Summary

Dependent Variable                  count
Number of Observations                250
Data Set                       MYLIB.FISH
Model                                 ZIP
Log Likelihood                      -1032
Number of Iterations                   13
Optimization Method          Quasi-Newton
AIC                                  2075
SBC                                  2096

Algorithm converged.

Parameter Estimates

Standard                 Approx
Parameter        DF        Estimate           Error    t Value    Pr > |t|

Intercept         1        2.431911        0.041271      58.93      <.0001
child             1       -1.042838        0.099988     -10.43      <.0001
camper 0          1       -0.834022        0.093627      -8.91      <.0001
camper 1          0               0               .        .         .
Inf_Intercept     1        1.297439        0.373850       3.47      0.0005
Inf_persons       1       -0.564347        0.162962      -3.46      0.0005


#### SAS Zero-inflated Poisson analysis using proc nlmixed

For those using a version of SAS prior to 9.2, a zero-inflated negative binomial model is doable, though significantly more difficult.  Please see this code fragment: Zero-inflated Poisson and Negative Binomial Using Proc Nlmixed.

#### Things to consider

• Since zip has both a count model and a logit model, each of the two models should have good predictors. The two models do not necessarily need to use the same predictors.
• Problems of perfect prediction, separation or partial separation can occur in the logistic part of the zero-inflated model.
• Count data often use exposure variables to indicate the number of times the event could have happened. You can incorporate exposure into your model by using the exposure() option.
• It is not recommended that zero-inflated poisson models be applied to small samples. What constitutes a small sample does not seem to be clearly defined in the literature.
• Pseudo-R-squared values differ from OLS R-squareds, please see FAQ: What are pseudo R-squareds? for a discussion on this issue.

#### References

• Cameron, A. Colin and Trivedi, P.K. (2009) Microeconometrics using stata. College Station, TX: Stata Press.
• Long, J. Scott (1997). Regression Models for Categorical and Limited Dependent Variables. Thousand Oaks, CA: Sage Publications.

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